.4 Contribution to the Geologic History of the Floridian Plateau. 137 



of course, is washed into the sea in suspension, where it is precipitated 

 on the bottom, increasing the quantity of the calcareous sediments. 



Another possible source of material is the region north of the west 

 coast of Florida Bay. Various streams, namely Caloosahatchee River, 

 a number of smaller streams emptying into the Gulf in the vicinity of 

 Thousand Islands, and others further south, cross areas underlain by 

 strata containing more or less lime. A portion of these waters may work 

 their way southward into Florida Bay and contribute to the supply of 

 sediment for that region. 



CALCIUM CARBONATE OF ORGANIC ORIGIN. 



A large proportion of the calcium carbonate of tliis region is of 

 organic origin. This is shown by the number of tests of various animals 

 found in almost any dredge haul, and those washed ashore, particularly 

 after storms. The principal constituent of this material is furnished by 

 marine moUusks. Foraminifera are important, as along the shores of 

 the keys one of the commonest organisms is Orbictdina adimca. Corals 

 have contributed to the calcareous material, but they are not very 

 abundant within the main line of the reefs and behind the keys. The 

 region between the keys and the main Florida shore is decidedly different 

 from that further to the west represented by the Marquesas and the 

 Tortugas. The limestone composing these two groups is very likely 

 almost entirely of organic origin. On the Tortugas, although corals con- 

 tribute a large proportion of the calcium carbonate, it appears that they 

 are probably secondary in importance to the mollusks. The breccia on 

 Loggerhead Key is largely formed of wave-tossed molluscan shells. 



Two additional sources may fm'nish calcareous sediment to the 

 bays and sounds. The first is one to which attention was first called by 

 Captain Hunt,' whose view was advocated by Mr. Agassiz in his "Three 

 Cruises of the Blake" (p. 57). 



As the prevailing direction of the winds and waves, both of trade 

 and hurricane origin, upon the Florida reef is from northeast to south- 

 east, they tend to pick up and carry behind the keys the loose sediment 

 previously prepared by the pounding of breakers ; and sediment once 

 having lodged behind there is not likely all to be transported back to 

 the sea by the ebbing of the tides, although, as will be shown later, the 

 outward flowing tidal currents in some instances build deltas at the 

 seaward end of passages between keys. 



The general character of coral reefs and the effect of the waves in 

 comminuting pieces of coral or the shells of other organisms have been so 

 frequently and fully described that it is superfluous to furnish a descrip- 

 tion here. 



r6sUM6 of sources of CALCAREOUS SEDIMENTS. 



Reviewing the sources of the calcareous sediment poured into the 

 bays and sounds we find: 



(i) That from the mainland and keys surrounding them. This 

 material is derived both by chemical denudation and mechani- 

 cal erosion. 



' Am. Jour, of Sci., 2d Series, vol. 35, p. 202, 1863. 



